Plant World

Soil, Gardening, and Farming

What do the numbers on a bag of fertilizer indicate?

Some gardeners make a simple taste and smell test to check the soil. Acid soil smells and tastes sour. Some put a soil sample in a jar of vinegar. If the vinegar starts to bubble, your soil has plenty of lime. If there are no bubbles, lime the soil with 4 ounces (113 grams) of lime for every square yard (0.84 square meter).

The three numbers, such as 15–20–15, refer to the percentages by weight of macronutrients found in the fertilizer. The first number stands for nitrogen, the second for phosphorus, and the third for potassium. In order to determine the actual amount of each element in the fertilizer, multiply the percentage by the fertilizer’s total weight in pounds. For example, in a 50-pound bag of 15–20–15, there are 7.5 pounds of nitrogen, 10 pounds of phosphorus, and 7.5 pounds of potassium. The remaining pounds are filler.